The aforementioned United States patent describes and illustrates a Prism Light Guide for transmitting or "piping" light from a central source to a remote location or locations. As used herein the term "light guide" means a light guide as described and claimed in the aforementioned United States patent. More particularly, the term "light guide" as used herein means a hollow structure comprised of sections having inner and outer surfaces which are in "octature" in that, over regions of each section which are not large relative to the thickness of the walls of the light guide in such regions, each section satisfies all of the following conditions:
1. All of the inner surfaces of each section are either perpendicular or parallel to one another;
2. All of the outer surfaces of each section are either perpendicular or parallel to one another; and,
3. Each of the inner surfaces is at a 45.degree. angle to each of the outer surfaces.
As explained in the aforementioned patent, light may be constrained to travel through such a light guide without escaping through the walls of the guide if the angle by which the light rays deviate from the longitudinal axis vector of the guide does not exceed a maximum angle which depends upon the refractive index of the light guide material and which can be shown to be 27.6.degree. for light guide material such as acrylic plastic having a refractive index of 1.5.
In many applications it is desirable that light escape from the light guide in a continuously controllable manner. In particular, it is often desirable that light escape from the guide at a rate which varies along the length of the guide, such that the amount of light escaping per unit length along the guide has a selected distribution of values. In many situations it will be desirable that the amount of light escaping, per unit length, from the guide is substantially constant along the length of the guide so as to provide uniform illumination along the guide. In other situations it may be desirable to vary the amount of light escaping, per unit length, from the guide in order to provide a gradient distribution of light illumination.
Broadly speaking, there are seven mechanisms which may cause light to escape from a light guide:
1. Angular imperfection of the prism light guide material used to construct the light guide.
2. Light scattering phenomena caused by the physical characteristics of the particular prism light guide material used to construct the light guide.
3. Deformation or destruction of the prisms which make up the prism light guide material.
4. Removal of prism light guide material from a portion of the light guide.
5. Non-linearity of the central path of the light guide.
6. Variability of the cross-section of the light guide.
7. Placement of substantially non-light absorptive materials within the light guide.
The first two mechanisms are intrinsic characteristics of the prism light guide material used to form the light guide and therefore are not subject to control in the manner contemplated by this application. The remaining five characteristics may however be controlled, either singly, or in combination with one another, to cause light to escape from the light guide at a rate which varies along the length of the light guide, such that the amount of light escaping, per unit length, along the light guide has a selected distribution of values.